That too, is a great question, and I am not just saying that.
When I was in highschool I had this phenomal idea that I could use raw water to run in engines. I understood and studied Diesel engines with great interest. I had my own set of experiments that I ran and actually got my lawnmower to run like a diesel engine (off of oil) for about 1 minute.
But these experiments were leading into a larger experiment, that of running this engine on water.
The theory was that the high compression event in a diesel engine raises the temperature of the air up to at least 1500 degrees F. That is more than sufficient to boil water I thought. Boiling water (ie steam) should just push the piston down.
I called it the Internal Steam Engine.
Sounds great in the talking, but when the engineering and physicist looks at it from an energy stand point: Uh NO, not such a good idea.
Why? Because water when it is introduced into this environment will stay water, because of the high pressure. And secondly, the energy that it is giving off as steam had to come from somewhere.
I know that sounds very technical, but the bottom line is that water can be used as a power source if it is used in the following ways:
-Energy Transfer Medium
-Chemical Reaction Mechanism
Engines by their nature are thermal devices. That means that they convert thermal energy into rotational energy, or kinetic energy. Steam is nothing but a heat transfer medium. The steam is water that has high energy in it. Steam expands, and because it expands the energy from expansion can be used to push on pistons or turbine blades.
But the water itself is not the fuel, it has to be invested with energy. For example in a nuclear power plant, the nuclear energy (which is heat energy primarily, not radioactive energy) is transfered into the water. The water is energized so much that it turns into steam. The steam then is used to push turbine blades. The Turbine then obviously rotates a generator, and then we get electricity (so that I can respond to this question.)
A simple example is water boiling on a stove. The kettle will spout out steam and actually push on a paper spinner wheel if given the chance. The steam itself however would not exist without the burner underneath it. Watch how fast the steam stops existing once the burner is shut off!
The other method is to break down water into Hydrogen and Oxygen. Fuel cells for example use catalysts to keep the Hydrogen-Oxygen reaction from becoming violent.
In fuel cells heat can be given off to run things. Also various types of battery devices (meaning chemical energy being converted into electricity) are used in these cells too.
Hydrogen can be burned in an engine, but because it is so volatile, and so reactive it is dangerous to deal with. For example, hydrogen is hard to contain in vessels because of its reactive nature with metals, it will literally corrode away the vessel (glorified bottle) that it is contained in.
So to answer your question, can water be used as a fuel, Yes and No. How much money do you have?
Practically speaking on a go kart, kind of expensive.
